Begin by preparing the crust. In a large bowl combine the lemon zest, sugar, and salt and pinch it together until fragrant (this helps release the oils from th zest). Add the flour and mix until combined.all of the dry ingredients.

Grate the butter on the largest hole setting of your grater over the bowl, mixing to coat the butter shards in the flour mixture every 10 seconds or so.

Add the cider vinegar and 5 tablespoons of water and mix the dough. If it stays in a clump when you squeeze it in your hand, it has enough water, if it falls apart, add more water until it stays together. Shape the dough into a ball and roll it out into an 11-inch disc. Cover with plastic wrap and refrigerate.

Preheat the oven to 400 degrees Fahrenheit. To make the filling, heat the strawberries, rhubarb, salt, and 3 tablespoons of the sugar in a large frying pan over low heat until softened slightly and the strawberries have begun to release their juices, about 8 to 10 minutes stirring every minute. Stir in the lemon juice and set aside.

Add the sugar to a large stainless steel frying pan over medium high heat (using a light colored pan will make it easier to see the color change of the sugar). Cook until the sugar has melted and caramelized and the mixture is a deep gold bubbling liquid, stirring often as the sugar melts. Remove from heat and stir in the butter and vanilla extract.

Pour the caramel mixture into a greased 10 or 11-inch skillet. Starting on the outside edge and working your way in, place the strawberries in the skillet, creating a circular pattern with all the pointy ends pointing towards the center. Scatter the chopped rhubarb and tarragon on top, reserving any liquid left from the rhubarb and strawberry pan. Place the crust over the skillet, using a spatula to tuck it down into the pan around the edges. Cut three 1-inch slits in the top to allow heat to escape before putting it in the oven.

Bake for 25 to 30 minutes of until the top of the crust is lightly golden and you can see juices bubbling up from around the edges. Remove from the oven and allow to cool for 20 minutes before attempting to flip the pan over onto the serving plate. Once flipped, gently remove the pan to display the filling inside. If any strawberries stick to the pan, just remove them and place them on a platter. Brush with the reserved cooking liquid from the strawberry and rhubarb pan and serve.